1. Field of the Invention
This invention relates to a limiting current type oxygen sensor, in particular, to a limiting current type oxygen sensor that can correctly measure oxygen concentration in a low concentration range even if applied voltage is unstable.
2. Description of the Related Art
The limiting current type oxygen sensor having a four-layer structure consisting of a porous diffusion layer for controlling gaseous diffusion rate, a cathode layer, a solid electrolyte layer and an anode layer can measure without reference gas and the like, output current linear to the gas concentration, and accurately measure the concentration from a low concentration level to a high concentration level. Therefore, a growing number of conventional concentration cell type sensors with conventional solid electrolyte layer are replaced by the limiting current type oxygen sensors.
FIGS. 1A and 1B show an example of the conventional limiting current type oxygen sensor. FIG. 1A is a top view, and FIG. 1B is a cross-sectional view taken on line L-L of FIG. 1A.
In this limiting current type oxygen sensor, a plane cathode layer 101 and a plane anode layer 103 are arranged on both sides of a solid electrolyte layer 102, and a porous diffusion layer 104 for controlling gaseous diffusion rate is arranged on the other side of the cathode layer 101. Additionally, the cathode layer 101 has a bonding pad 105 for connecting electrically to an external lead wire. A part of the bonding pad 105 is covered with the solid electrolyte layer 102, and the other part of the bonding pad 105 is exposed to an atmosphere.
A principle of the limit current type oxygen sensor will be described below with reference to FIG. 2. By applying voltage between the cathode layer 101 and the anode layer 103 via the solid electrolyte layer 102, firstly oxygen gas at the cathode layer 101 is ionized. Then, the ionized oxygen is shifted to the anode layer 103 through the solid electrolyte layer 102 previously heated to a suitable temperature for ion conduction by a heater (not shown). Then, this ionized oxygen returns to the oxygen gas at an interface between the anode layer 103 and the solid electrolyte layer 102. At this moment, a current corresponding linearly to the shifting ionized oxygen, namely linear to the oxygen gas supplied to the cathode layer 101, flows from the anode layer 103 to the cathode layer 101.
In this case, as indicated by a bold arrow in FIG. 2, the oxygen gas is supplied to the cathode layer 101, through the porous diffusion layer for controlling gaseous diffusion rate 104 (hereafter referred to as a “diffusion layer”) from the atmosphere. The supply is regulated by the diffusion layer 104 structured optimally, and proportional to the oxygen gas concentration in the atmosphere. Therefore, the oxygen concentration in the atmosphere is known by only measuring the current from the anode layer 103 to the cathode layer 101 as the limiting current.
In a sensor having a common form as shown in FIG. 2, FIG. 3 shows a relation diagram between voltage, applied the cathode layer 101 and the anode layer 103, and current (sensor output) at 20.6% of oxygen concentration in the atmosphere surrounding the sensor.
In FIG. 3, a region indicated by “a” is a resistance region, a region indicated by “b” is a limiting current region, and a region indicated by “c” is a overcurrent region. In the limiting current region as the region b, the sensor output (current) is regarded as independent of the applied voltage, and a relation between the oxygen concentration and the sensor output (current) is regarded as linear.
However, as shown by a broken-line arrow in FIG. 2, the oxygen gas supplied to the cathode layer 101 through the porous diffusion layer 104 is supplied through not only a backside of the porous diffusion layer 104 corresponding to the cathode layer 101 originally expected, but also a side surface of the porous diffusion layer 104 or a surface exposed at the cathode layer 101 side. Therefore, a characteristic curve of the relation at the region b in FIG. 3 has a positive slope and linearity of the relation between the oxygen concentration and the output is degraded. In this case, measurement accuracy is influenced greatly by voltage drift. Otherwise, a linearity compensation circuit is needed to maintain the measurement accuracy. Resultingly this increases cost and power consumption of the sensor, and creates other problems.
To solve these problems, a method of preventing inflow of the oxygen from an unnecessary part by covering parts other than an originally expected passage of oxygen with a gas barrier layer such as glass or gas barrier alumina is proposed by Japanese Utility Model Application Laid-Open No. Show 61-97753, Japanese Utility Model Application Laid-open No. Hei 3-104849 and the like.
However, according to these, heat capacity and power consumption of the sensor is increased. Therefore, a load of the heater for heating the sensor to its operating temperature, namely a temperature suitable for oxygen ion conduction in the solid electrolyte layer, is increased and a lifetime of the heater is decreased. Additionally, required time from power-on to the sensor operating temperature being stabilized by heating is increased. Therefore, there are problems such as time-consuming measurement or considerable power consumption in an intermittent measurement for use in low power for such as battery powered applications. Further, there are problems such as a short lifetime or a lack of reliability of the gas barrier layer because of peeling or cracking of the gas barrier layer caused by a difference among coefficients of thermal expansion and a temperature difference at the intermittent measurement.
Moreover, according to the invention as described in Japanese Utility Model Application Laid-open No. Hei 3-104849, there is provided a limiting current type oxygen sensor which can measure oxygen gas precisely from low to high concentration of the oxygen gas. However, since alumina as a gas barrier layer is needed to deposit on either side surface of the sensor, film deposition process becomes complicated, and as a result, cost of the sensor may be increased.